The present invention relates to a three-dimensional, photocatalytic filter apparatus for removing organic or inorganic pollutants in a gas or liquid stream by using a semiconductor photocatalyst.
Ever increasing use of petrochemicals has caused serious environmental pollution problems. To solve the pollution problems, proposal has been made to purify a fluid by the photocatalytic oxidative decomposition of pollutants with a semiconductor photocatalyst. For example, an apparatus or instrument having a porous substrate supporting a semiconductor photocatalyst is placed in a flow path of a fluid containing pollutants to decompose the pollutants with the semiconductor photocatalyst.
In this method, to enhance the photocatalytic efficiency, it is needed to increase the photocatalytic surface area and to sufficiently activate the semiconductor photocatalyst with light having a wavelength capable of activating the photocatalyst. Several proposals have been made to increase a photocatalytic surface area to increase the opportunity of contact between pollutants and a semiconductor photocatalyst, and to form a photocatalyst layer with increased photocatalytic surface area. See Japanese Patent Laid-Open Nos. 5-309267, 8-196903, etc.
Well known in the art to increase the photocatalytic surface area is the formation of a photocatalyst layer on a porous substrate having a large surface area such as a layer of bonded fine particles, a nonwoven fabric, glass wool, a spongy substrate, etc. However, when a plurality of porous filter plates are arranged in a filter apparatus, a semiconductor photocatalyst deposited on all the porous substrates cannot be activated, because each porous substrate absorbs or cuts an activating light. If a layer of silica glass powder, soda glass powder or glass wool were used as the porous substrate, the activating light would reach comparatively deeper inside the porous substrate. However, such a porous substrate cannot be formed larger than about 30 mm in diameter due to its poor mechanical strength. Also, to achieve sufficient light-absorbing efficiency, the porous substrate should be as thin as about 2 mm or less. Therefore, this porous substrate is not promising in practical applications.
Some proposals also have been made to improve the shape and arrangement of a photocatalytic filter. For example, Japanese Patent Laid-Open No. 7-108138 discloses a photocatalytic filter apparatus comprising photocatalyst-supporting thin plates arranged in a blind form in a flow path of a fluid. Japanese Patent Laid-Open No. 8-121827 discloses a photocatalytic filter apparatus comprising a photocatalyst-supporting, corrugated nonwoven fabric on both sides of an ultraviolet lamp in a flow path of a fluid. Japanese Patent Laid-Open No. 9-187491 discloses a photocatalytic filter apparatus placed in a flow path of a fluid, the apparatus comprising a plurality of porous substrates each supporting a photocatalyst on both surfaces and radially fixed around a light source. Japanese Patent Laid-Open No. 9-248426 discloses a movable, photocatalytic filter apparatus comprising a photocatalyst layer formed on a convex member for conveying or stirring a fluid, and an ultraviolet source placed inside the convex member.
When a fluid containing pollutants is brought into contact with a semiconductor photocatalyst, active radicals generated by the semiconductor photocatalyst oxidatively decompose pollutants. If the total amount of pollutants exceeds a capacity of the conventional photocatalytic filter apparatuses, the pollutants would not fully come into contact with the photocatalyst, suffering disadvantages that it takes a lot of time to ensure contact of all the pollutants with the photocatalyst. It is difficult to increase a flow rate of the fluid in the conventional photocatalytic filter apparatuses, while enhancing decomposition efficiency. To enhance photocatalytic efficiency, it is necessary to use a suitable activating light that can sufficiently irradiate the semiconductor photocatalyst. Therefore, the development of a photocatalytic filter apparatus capable of efficiently irradiating the semiconductor photocatalyst has been desired.
In addition, intermediate products formed in the oxidative decomposition of nitrogen compounds, sulfur compounds, chlorine compounds, etc. tend to stick to a surface of the semiconductor photocatalyst, making it necessary to remove the intermediate products from the semiconductor photocatalyst at regular intervals. Accordingly, the photocatalytic filter apparatus is required to have a structure capable of being easily cleaned to prevent reducing a decomposition capacity thereof.